This invention relates to a rotary gas compressor. More particularly, it relates to a rotary gas compressor in which interaction between a helix and a liquid ring takes place for obtaining a compression ratio that is perfectly defined and characteristic for each construction. The use of a fluid as a propelling means is known for rotary compressors with blades or screws. So-called liquid ring compressors are also known. These compressors have various drawbacks. For instance, blade compressors when they have a defined compression ratio for a specific pressure, show wear and tear and a moderate efficiency. The screw compressors in an oil bath require a high speed and present an occasional mixture of the gas with the oil. The liquid ring compressors have a low efficiency and suffer losses by attrition, which are very high and produced by the attrition between said liquid ring and walls of the compressor.